In recent years, there have been demands for devices having smaller thickness, lighter weight, and even flexibility in association with rapid progress in electronics such as display devices (e.g., liquid crystal display devices, organic EL display devices, and electronic paper), solar cells, and touch panels. To meet the demands, replacement of glass substrates used in such devices with plastic film substrates capable of achieving smaller thickness, lighter weight, and flexibility has been studied.
These devices have various electronic elements including, for example, a thin film transistor and a transparent electrode formed on the substrates thereof. These electronic elements need to be formed through a high temperature process. Therefore, the plastic film substrates need to have a sufficient heat resistance adaptable to the high temperature process.
Further, the plastic film substrates need to have transparency for the exit of light emitted from display elements through the plastic film substrates (for example, when the plastic film substrates are used for a bottom-emission-type organic EL display). In particular, the plastic film substrates are required to have a high light transmittance in a wavelength range of 400 nm or less within a visible range. Moreover, for the passage of light through a phase difference film and a polarizing plate (provided in, for example, a liquid crystal display and a touch panel), the phase difference film and the polarizing plate are required to have not only transparency but also a high optical isotropy.
Fabrication processes of these devices can be classified into a roll-to-roll type fabrication process and a batch type fabrication process. The use of the roll-to-roll fabrication process requires a new facility and further need to overcome several problems resulting from a rotation and a contact. On the other hand, the batch type fabrication process is a process of applying a coating resin solution on a glass substrate, drying a coating for substrate formation, and then peeling the substrate thus formed. Therefore, the batch type fabrication process can be performed with use of a process facility for a glass substrate such as a current TFT. For this reason, the batch type fabrication process is superior in terms of cost.
Given the circumstances, there has been a strong demand for the development of a material that is adaptable to the existing batch process, excellent in heat resistance, transparency, and optical isotropy, and soluble in a general-purpose organic solvent.
As a material that meets the above demand, a polyimide material known as being excellent in heat resistance has been studied. In a case where a polyimide having a high transparency is to be obtained, an alicyclic monomer is typically used (Patent Documents 1 and 2). Meanwhile, a polyimide having a fluorene structure is known as being effective in exhibiting solubility, controlling optical properties, and offering a stable adhesiveness with time (Patent Documents 3 and 4)